Topic: '76 CB400F tail fuse woes

[Matt Chapter]

I'm trying to get my CB400F back on the road, and the last thing I need to get worked out is the running light.  The fuse blows consistently, although not immediately, and is melted for a decent section of the middle.

I have already replaced the start/stop switch and the ignition switch.  I have also cleaned the fuse connections and the rear brake switch connectors.  I have continuity on the tail light wires from the rear connections to the tail light.

I'm sort of at a loss, the tail light circuit is super simple and I can't seem to find the fault.

[Proto]

I had a similar problem with the main fuse on my 76 CB750F1.  All my connections were good but every once in a while the fuse would blow.  I then noticed that even if it did not blow, it would be real hot. 

Doing a search on this forum, I found that my problem was probably a bad fuse holder.  I learned that once the clips that hold those glass tube type fuses in place get over-heated, they loose their ability to tightly hold the fuse for a good connection.  The info also stated that you should not be able to easily remove the fuses from a good fuse box with just your fingers.  Heck, I pop all my fuses out easily!  Time for a new fuse box.

I did not want to continue using glass fuses, so I went to a block using those mini-blade type fuses.  The smallest one that NAPA had held 4 fuses ($15) and fit into the space previously occupied by the old 3 glass fuse box.  Been riding the bike for over a week now and the fuse is cool as a cucumber!  (I even dropped the amp rating of main fuse back down to the recommended 15 amps.  I had stupidly put a 20 amp glass fuse in the old block, but got scared when I felt how hot it had got!) 

Don't know if this helps you but it sure fixed my fuse problem.

[camelman]

Proto makes a great point. Also, check for frayed wires. Those seats can tear up a loose wire very quickly.

[mronegear]

When a fuse melts in the middle, its normally not a bad fuse holder, it would
melt at the end because of the heat.
When my Taillight fuse blew constantly, i forgot that green wire is ground 

[Bodi]

The tail (and front "parking" light if you have one) circuit is unusually complex. The ignition switch selects where it gets power from, in "ON" it is powered from the TAIL fuse. in "P" it's powered directly from the MAIN fuse. I would install a tail light bulb (or similar wattage) in place of the TAIL fuse, switch the kill switch off and remove the HEAD fuse to reduce current draw, and turn the key ON. Then if the "fuse" bulb burns bright, you have a "short" somewhere. Disconnect things starting from the tail light to see what it is. Note the tail circuit also shows up in the headlight shell (for that front parking light) and you may have that connector touching ground intermittently (on bumps?) which will burn the fuse immediately.